1. Field of the Invention
The invention relates to a friction brake system with self-boosting, and more particularly to such brake systems which are intended for use in motor vehicles.
2. Description of the Prior Art
Known friction brakes with self boosting are often embodied as disk brakes having electromechanical actuation. The known brakes have a friction brake lining, which for braking can be pressed against a brake body to be braked, such as a brake disk. To achieve self-boosting, wedge mechanisms are known, which have one wedge on a side of the friction brake lining remote from the brake body and braced on a ramp. The ramp extends obliquely to the brake body, preferably at an acute angle; it forms an abutment on which the friction brake lining is braced at a bracing angle, which effects the self-boosting, obliquely to a normal of the brake body. The normal direction refers to a surface of the brake body against which the friction brake lining is pressed upon actuation of the friction brake. The bracing angle is the same size as a wedge angle, or an angle below which the ramp forming the abutment leads to the brake body. The angle at which the ramp leads to the brake body can vary over the course of the ramp, as a result of which the self-boosting also varies. In that case, the ramp is then no longer straight but instead is for instance curved.
For actuation, the friction brake lining is displaced by an actuating force in the direction of a tapering wedge gap between the abutment and the (rotating) brake body moving in the same direction, and as a result the friction brake lining comes into contact with the brake body. A frictional force exerted by the rotating brake body on the friction brake lining urges the latter in the direction of the tapering wedge gap, as a result of which an additional contact pressure of the friction brake lining against the brake body results by way of the abutment. The contact pressure of the friction brake lining against the brake body is thus effected only in part by an actuating device of the friction brake, and for the remainder is effected by a frictional force exerted by the rotating brake body on the friction brake lining. Such friction brakes are known for instance from German Patent DE 198 19 564 C2 or German Patent DE-PS 1 096 220.
However, the invention is not limited to friction brakes with wedge mechanisms for effecting the self-boosting, but instead can in principle be employed equally well in other self-boosting mechanisms, such as a lever system in which the friction brake lining is braced on an abutment via an obliquely placed lever. The invention is not limited to disk brakes, either, but can also be used with other types of brake, such as a drum brake, although theoretically that can be considered an analog to a lever mechanism.
In the aforementioned DE 198 19 564 C2, a number of friction brake linings is distributed over the circumference on an annular disk, on whose side remote from the brake disk wedges are mounted for effecting the self-boosting. For actuation, an electric motor is provided, which has a gear wheel that meshes with an internal toothing of the annular disk.
For the self-boosting C* of a friction brake of the type defined at the outset, the following equation applies:       C    *    =                    F        u                    F        app              =                  2        ⁢        μ        ⁢                                   ⁢                  (                                    sin              ⁢                                                           ⁢              β                        +                          tan              ⁢                                                           ⁢                              α                ·                cos                            ⁢                                                           ⁢              β                                )                                      tan          ⁢                                           ⁢          α                -        μ            in which FU is the circumferential force, Fapp is the actuating force exerted on the friction brake lining, μ is the coefficient of friction, α is the bracing angle or wedge angle, and β is the force introduction angle at which the actuating force acts on the friction brake lining. The factor 2 is due to an engagement of both friction brake linings on both sides of a brake disk of a disk brake. If there is only one-sided engagement or a different brake design, the factor is 1. The above equation thus results in a maximum force boosting, that is, a maximum circumferential force FU for a given actuating force Fapp as follows:β=90°−α
This should not be mistaken for the self-boosting of the brake that is dependent on the bracing angle α, which is to be assumed as given in this case. The above equation indicates the most favorable force introduction angle β; that is, if the actuating device exerts the actuating force on the friction brake lining at the force introduction angle β indicated by the above equation, the result is a maximum circumferential force FU, that is, a maximum braking of the brake body for a given actuating force.